This invention relates generally to thin film transistors, active matrix assemblies including thin film transistors, and more particularly to improved thin film transistors including polycrystalline silicon or amorphous silicon thin films and active matrix assemblies including the thin film transistors for providing improved thin-type displays.
Applications of thin film transistor technology are wide-spread, including their use as active switching elements in thin matrix panels and three-dimensional integrated circuits and the like. A recurring problem encountered when using thin film transistors is leakage current of the transistor when it is in the OFF state. In the conventional MOS type transistor utilizing monocrystalline silicon, a PN junction is utilized to decrease OFF current where a P-type substrate is used for N-channel and N-type substrate is used for P-channel. When using prior art polycrystalline silicon transistors, the formation of an effective PN junction cannot be attained and hence the OFF current cannot be decreased enough to allow its use in a matrix display. This problem is particularly undesirable when the transistor is being used as a switching element in, for example, an active matrix panel. A MOS transistor with a poly-crystalline-silicon film channel region of approximately 2 microns thickness is disclosed in Solid-State Electronics, 1972, Vol. 15, pp. 789-799. The authors reported on field-effects in polycrystalline-silicon films and threshold voltages of doped films and concluded that these MOS transistors would find limited practical application.
When a thin film transistor (TFT) is used as a switching element in a matrix-type arrangement in a liquid crystal display (LCD) device it selects the data signal to be applied to the liquid crystal material. In such construction, the TFT must have the following characteristics:
(1) Permit enough current to flow into a condenser for charging when the TFT is in the ON state;
(2) Exhibit insignificant current flow in an electrode when the TFT is in the OFF state; and
(3) Show stabilized, reproducible performance and reliability.
Requirement (1) relates to the TFT's inputting a data signal into a condenser. A TFT must accept a large amount of current flow for a short time so as to completely input the data signal to the condenser, since the quality of a liquid crystal display depends in part on the capacitance of the condenser. This amount of current flow (hereinafter referred to as "ON current") is determined by the capacitance of the condenser and the time elapsed in writing the data signal to the condenser. Compliance with condition (1) depends largely on the TFT's size (especially channel length and width), construction, manufacturing process and input voltage to its gate. A TFT composed of polycrystalline silicon is capable of carrying a sufficient amount of ON current, and satisfying the requirement (1), since the polycrystalline silicon has a large carrier mobility comparing with that of amorphous semiconductors.
Requirement (2) relates to the holding time of written data in a condenser. Generally, written data in a condenser must be kept for an extremely long time compared with the writing time of that data to the condenser. Since the capacitance of the condenser is generally a very small value, such as 1 pF, if there is even a small amount of leakage current ("OFF current") at the TFT, during the OFF state, then the driving voltage of the electrode sharply drops to the level of voltage of the data signal time. As a result, the written data signal cannot be held properly at a condenser during the OFF state, and image sharpness is lost. This problem has been especially so in the case of prior art polycrystalline silicon TFTs, where deep and shallow trap levels are unevenly distributed in a crystal grain, allowing leakage current to flow via these trap levels, as discussed in the above-identified article in Solid State Electronics.
The requirement for insignificant current flow in the OFF state is necessary in other applications of TFTs, for example, in logic circuits using TFTs where stationary current increases and in memory circuits using TFTs.
Requirement (3) relates to stability, reproducibility and reliability of thin film transistor characteristics. Generally, several tens of thousands of thin film transistors are formed on one active matrix substrate and all of them must have uniform characteristics and superior reproducibility with no dispersion among manufacturing lots and remain stable and delivery reliable performance for an indefinitely long term.
Conventional thin film transistors as active elements on a substrate include a compound semiconductor, such as calcium selenium, and the like, or non-crystalline semiconductor, such as amorphous silicon, and the like, as the semiconductor thin film. However, these TFTs cannot satisfy all of the three above-mentioned requirements. For example, a TFT including a semiconductor film satisfies requirement (1) due to the high value of carrier mobility, but cannot meet requirements (2) and (3), since this type of semiconductor exhibits poor stability and reproducibility. A non-crystalline semiconductor has a low carrier mobility, and hence substantially small ON current flows. As noted above, an active matrix panel utilizing such a thin film transistor on a substrate did not exhibit sufficiently good characteristics for providing satisfactory image quality of the above matrix panel.
Accordingly, it is desirable to provide improved thin film transistors having reduced leakage current in the OFF state that overcome deficiencies found in the prior art.